1. Field of the Invention
The present invention generally relates to the art of electrochemical cells and, more particularly, to a lithium-containing cell with improved voltage delay characteristics. In general, it has been discovered that exposing lithium to gaseous carbon dioxide improves the safety and voltage delay characteristics of a cell containing the active material, regardless of whether the cell is of a primary or of a secondary chemistry. The present invention is particularly applicable to lithium oxyhalide cells.
2. Prior Art
Primary lithium oxyhalide cells are used extensively in applications requiring high gravimetric and volumetric energy density. Among the many sizes and chemistries available, cells can be developed for low rate or high rate applications and to operate from temperatures as low as xe2x88x9270xc2x0 C. to as high as 200xc2x0 C. The anode material usually consists of lithium or lithium alloyed with various elements such as aluminum, magnesium or boron and the cathode usually consists of some form of carbon which is held together using a suitable binder. The electrolyte generally consists of a solvent system of thionyl chloride, phosphoryl chloride or sulfuryl chloride. Often, additional compounds or interhalogen compounds such as sulfur dioxide, chlorine, bromine, bromine chloride and others may be dissolved therein to modify the cell for a particular purpose, such as extending the operating rate or temperature of the cell. Electrolyte salts are also added to the solvent system to assist in ionic transfer during cell discharge. Such salts may include lithium chloride in combination with aluminum trichloride or gallium trichloride. Lithium tetrachloroaluminate salt (LAC) or lithium tetrachlorogallate salt (LGC) is then formed in-situ. Typically used catholytes include chlorinated sulfuryl chloride (CSC) having either LAC or LGC dissolved therein. These systems are commonly referred to as LAC/CSC and LGC/CSC.
While lithium oxyhalide cells are well known for their high energy and power density, there are some drawbacks to their use in particular situations. Unlike other pulse dischargeable lithium primary cells containing solid cathode systems and organic-based electrolytes, such as the lithium/silver vanadium oxide system (Li/SVO) or the lithium/manganese dioxide system (Li/MnO2), lithium oxyhalide cells are more prone to exhibit voltage delay under some use conditions.
The voltage delay phenomenon manifests itself as a rapid decrease in discharge voltage when an external load is placed upon the cell or battery, such as during the application of a short duration pulse or during a pulse train. Voltage delay can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied. FIG. 1 is a graph showing an illustrative discharge curve 10 as the voltage response of a cell that exhibits both forms of voltage delay. In extreme cases, the voltage may drop so low that the cell or battery is rendered useless. Generally, the voltage recovers or rises to an acceptable level over a period of several seconds or minutes. Especially in a lithium oxyhalide cell, it is well known that the voltage delay phenomenon becomes more problematic as the cell ages, as the storage temperature increases, as the discharge rate increases and as the discharge temperature of the cell decreases.
The voltage response of a pulse dischargeable cell which does not exhibit voltage delay during the application of a short duration pulse or pulse train has distinct features. First, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse, and second, the minimum potential of the first pulse in a series of pules is higher than the minimum potential of the last pulse. FIG. 2 is a graph showing an illustrative discharge curve 12 as a typical or xe2x80x9cidealxe2x80x9d response of a cell during the application of a series of pulses as a pulse train that does not exhibit voltage delay.
In the lithium oxyhalide chemistry, the voltage delay phenomenon is primarily attributed to a passivation layer which forms on the lithium anode as the catholyte is filled into the cell. Prior to filling, the passivation layer primarily consists of oxygenated surface species formed from reaction of the anode with oxygen in the dry air cell assembly environment. When the cell casing is filled with the catholyte solution, a more robust passivation layer consisting of lithium chloride and various electrolyte salt species is formed. This passivation layer prevents the cell from internally short circuiting since the electrolyte itself is consumed during discharge; however, it also causes an additional resistance within the cell which must be overcome during discharge. It is generally recognized that modification of the lithium passivation layer is critical to improving the voltage delay characteristics of lithium oxyhalide cells.
It is a premise of the present invention that lithium carbonate possesses many of the same properties as lithium chloride. Lithium chloride is the predominant compound of the passivation layer in a LAC/CSC or LGC/CSC system. Among these, lithium carbonate is stable throughout the temperature range in which lithium oxyhalide cells are used, it is electrically non-conductive and ionically conductive, and it is non-reactive with strong oxidizing agents such as thionyl chloride, phosphoryl chloride and sulfuryl chloride.
Accordingly, the present invention is directed to providing a lithium carbonate passivation layer on lithium active material through exposure to gaseous carbon dioxide prior to cell assembly. This results in an electrochemical cell which possesses improved safety and voltage delay characteristics in comparison to prior art cells where lithium is not so exposed. These benefits are realized in cells of a primary chemistry having a solid cathode or of an oxyhalide type, or a cell of a secondary chemistry.
In the preferred oxyhalide chemistry, the present invention provides an electrochemical cell of high energy density including an alkali metal anode, a cathode current collector of electrically conductive and/or electroactive material and an tonically conductive catholyte solution operatively associated with the anode and the cathode current collector. The catholyte consists essentially of a first depolarizer component selected from the group consisting of free halogens, interhalogens and mixtures thereof dissolved in a second depolarizer component in the form of a nonaqueous solvent or a mixture of nonaqueous solvents. A metal salt is dissolved in the catholyte solution to enhance the ionic conductivity thereof. The preferred active material for the anode is lithium, or an alloy thereof, that has been exposed to gaseous carbon dioxide prior to cell fabrication, and the preferred electrically conductive material of the cathode comprises a carbonaceous material.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description taken in conjunction with the accompanying drawings.